1. Field of the Invention
This invention relates to cardiac pulse rate monitoring devices, in general, and, more particularly, to a novel apparatus including a self-contained pump assembly for selectively positioning sensors in such monitoring devices.
2. Prior Art
Health and fitness is a relatively common concern and goal of many persons. One method of fulfilling these objectives is exercise. Electronic equipment for the monitoring of a person's heart pulse activity is employed in health care institutions and by persons during athletic training.
One important measurement parameter is the rate of occurrence of heart pulsations. In healthy persons, the pulse rate is substantially uniform throughout the duration of a person's normal activity. However, the rate varies with changes in the person's activity when the heart may be called upon to pump at a higher or lower rate. The rate of pulse change during increasing or decreasing activity is directly related to a person's physical condition.
Thus, it must be appreciated that a device providing an accurate measurement of pulse rate is most useful during athletic training and for the detection and treatment of disease. Heart rate monitoring can be used to guide cardiovascular intensity to achieve maximum fitness results within any aerobic exercise regimen. The measurement of heart pulse rate is usually accomplished by an electronic unit worn by the individual. These units are worn on various parts of the body, most typically on the wrist. However, wearing the unit on the wrist usually necessitates the use of complex miniature electronic equipment.
That is, accurate measurement of an active person's pulse rate at the wrist is a complex process due to the artifacts produced by body motion. These artifacts are produced concurrent with the heart pulse and are, typically, detected by the heart pulse sensor as noise. However, in many cases, the artifacts produce signals of sufficient amplitude to completely mask the desired heart pulse signal. In order to mitigate the effects of these body artifacts, it is necessary to filter out and electrically cancel as much of the noise signal occurring in the heart pulse frequency band as possible while retaining the desired pulse signal.
This problem must be dealt with effectively over a considerable signal-to-noise ratio range. In some extreme cases, the signal-to-noise ratio will become negative even with very effective cancellation techniques. When it is no longer possible to reliably detect the heart pulse rate, it is necessary that no attempt is made to display a heart rate reading due to the high probability of introducing inaccuracies. It is better to store and display the last good reading until the severe noise condition is over and an accurate reading can be made.
However, during monitoring sequences, it is important that the user be able to frequently receive accurate updates of the heart pulse rate. It has been demonstrated that this should occur no more frequently than every five seconds with an update every ten seconds seeming to be optimal. This is important to the user since even in situations where violent physical activity is creating body artifact noise in excess of what can be tolerated by the system, only a short, relatively still period would be required to provide an updated readout of pulse rate.
Several devices have been proposed for providing a wrist watch type of heart pulse monitor. One such device is the digital plethysmography described in Prinz (U.S. Pat. No. 4,120,269), which customarily utilizes an infrared light transducer. Other devices have proposed using piezoelectric or other pressure sensitive transducers, such as in Stupay (U.S. Pat. No. 4,059,118), which uses an actuator pin pressing against a piezoelectric crystal.
Typically, such devices tend to have several shortcomings. Devices using optical transducers, such as the digital plethysmographies, consume substantial power in the light emitting elements and, thus, use up battery life rapidly. Devices using piezoelectric transducers, such as Stupay, typically devote little attention to the substantial noise problems that attend the use of such transducers in this application.
When such a pulse rate monitor is mounted on the wearer's wrist, the pulse signal is, to a significant extent, masked by the concurrent noise signals generated due to body motions. The mechanical transducer responds to, but does not distinguish between, pressure from the wearer's pulse beat or motion from walking, arm swinging and the like. The latter is noise insofar as pulse measurement is concerned. Thus, the user of the piezoelectric transducer system must remain quiet to avoid noise input" during the period in which the pulse rate is being measured.
Also, if the piezoelectric transducer is not mounted directly over the artery of the user, the pulse signal measured by the device will be significantly reduced in amplitude. This signal is, thus, even more likely to be masked by noise. Typically, noise signals may be as high as 1.0 volt, while the pulse signal may be approximately 0.1 volt. Prior art wrist pulse rate monitors employing piezoelectric transducers have been inaccurate because of this unfavorable signal-to-noise ratio. Thus, Cramer (U.S. Pat. No. 4,224,948) teaches that when a piezoelectric sensor is used, the monitor must be worn on the volar surface of the wrist but lateral to the tendon chord bundles so as to obtain a pulse reading from the radial artery in the subpollex depression. In this case, the sensors must be forced into the flesh of the wrist for a reading which may be uncomfortable.
Albert (U.S. Pat. No. 4,409,983) uses a complex arrangement of piezo sensors to develop relatively noise-free signals which are presented to the input of a microprocessor. This device employs a piezo sensor which is operated by providing a bending force to one end of the sensor element. This bending interaction is accomplished by using small pins pushing against the ends of the sensors with coil springs used to dampen the high frequency noise products. Algebraic analog signal summing is used to create relatively noise-free signals at the input to the microprocessor in the form of an electrical pulse string having the same rate as the heart pulse. While this device may reduce the attendant body noise, the complexity of the sensor system makes this arrangement impractical for mass production.
Monitoring systems using optical sensors to detect the heart rate pulse at the radial artery on the wrist tend to have many of the same body motion related problems as the piezo sensor systems. Added to the motion-induced noise problems is the introduction of noise artifacts that are caused by ambient light conditions. These noise sources can be any electrical or natural light sources such as the sun. However, optical sensors do not tend to detect body transmitted acoustical noise. An effective method of dealing with these noise sources is necessary in order to make accurate heart pulse rate readings when the body is in motion or exposed to changing lighting conditions.